Plastics have developed along with the advance of chemistry. Nowadays, plastics find use in various fields as a highly functional material substituting metal, and have become established as an indispensable material in the industry. The use of plastics is expanding from daily necessities to industries and atomic, space, and ocean technologies. Plastics are incomparable with other materials because of their characteristic properties and potential, such as light weight, good moldability for complicatedly shaped products, good corrosion resistance, and good chemical resistance. On the other hand, they are inferior in heat resistance to metal. In order to overcome such shortcomings, new polymers having good heat resistance and high strength and elastic modulus have been developed in the 1960s based on the past research concerning polymer science. The thus developed polymers are called engineering plastics, which are characterized by a heat distortion temperature higher than 100° C., a tensile strength higher than 60 MPa, and an elastic modulus higher than 2 GPa.
There has recently arisen a necessity for development of new plastics compatible with environment (i.e., degradable and non-toxic) to cope with the issues of environmental destruction and resource depletion. Plant-derived polymers (called green polymer or green-based polymer) are attracting attention. One of their typical examples is polylactic acid. However, it is not satisfactory in strength and heat resistance.
A liquid-crystalline polyester was developed in 1976. It is the first liquid-crystalline engineering plastics ever produced by modifying polyethylene terephthalate (PET) with p-hydroxybenzoic acid (PHB) for improvement in heat resistance. This development led to exploitation of new products such as liquid-crystalline polyarylate (types I and II). Nevertheless, nothing has been reported so far about engineering plastics that solve the foregoing problems.
With the foregoing in mind, M. Akashi et al. attempted to develop new liquid-crystalline engineering plastics by paying attention to 4-hydroxycinnamic acid (4HCA) as the plant-derived reactive rigid substance. Up to that time, there had been no report about 4HCA homopolymer (or poly-4HCA), except for those about synthesis and granulation. Akashi et al. found for the first time that poly-4HCA (belonging to polyesters derived from nature) exhibits nematic liquid-crystal. This polymer is reactive with light and compatible with living bodies, and it also has good heat resistance required of engineering plastics. However, it has a disadvantage of being brittle and poor in solubility and processability, presumably due to low molecular weight and rigid skeleton.
In order to address this problem, they adopted an idea of copolymerizing with a natural material that imparts flexibility to the skeleton of poly-4HCA and turned their attention to 3,4-dihydroxycinnamic acid (caffeic acid) (DHCA) which is a 4HCA derivative. This copolymer is obtained by polycondensation with heating at 200° C. for 6 hours in the presence of acetic anhydride (as an ester exchange agent) and sodium acetate (as a catalyst). Despite introduction of DHCA, the copolymer remains solid up to 25° C. and becomes fluid upon heating, with an apparent band pattern showing. This phenomenon proves the copolymer to be a liquid crystal. The liquid-crystallizing temperature falls to 150° C. and the weight-decreasing temperature exceeds 300° C. with the increasing amount of DHCA. The resulting copolymer has a broad liquid-crystalline temperature range and is easy to handle. Moreover, it has a high molecular weight essential for strength and elastic modulus. A compression test show that, in regard to strength and elastic modulus, it exhibits a high Young's modulus and breaking strength comparable to polycarbonate (which is a typical engineering plastics) when the content of DHCA is 50 to 100 mol %, although it depends on the composition ratio of copolymer. See Non-Patent Document 1. Unfortunately, this copolymer is rigid but hard and brittle (lacking flexibility) and slow in degradation.    Non-Patent Document 1: Engineering Plastics of Environmental Recycling Type, by M. Akashi, Journal of the Institute of Polymer Science “Kobunshi,” November.